A randomized, double-blinded, placebo-controlled study to evaluate the effect of spironolactone in addition to conventional treatment compared with placebo in patients with paroxysmal and persistent atrial fibrillation with preserved left ventricular ejection fraction by T1 mapping, structure and function of left atrium and ventricle assessed by transthoracic echocardiography and cardiac magnetic resonance (CMR), the number of recurrent episodes of atrial fibrillation and biomarkers measured in blood.